First writing some sentences in the conlang, then define grammar and vocabulary afterwards?

Or write some grammar, then try to make sentences and just see what kind of vocabulary you get?

Is a full grammar system needed first(ly?) to make a decent conlang or not?

I tend to make a base for a conlang, then find myself out of inspiration, put it away for some time, and forget about it...

I would definitely start with the phonology and up, but I guess you could just write down random sentences and then draw out the grammar, effectively going "backwards". Much harder though, but it's possible!

What works for me personally (I'm not an expert myself though) - 1. Play around with phonology first, decide what sounds you like. Then add any extra dimensions (e.g. tone/pitch, voiced/voiceless, vowel length) Then decide on syllable structure and stress.2. Decide on how to write the conlang - firstly in the Latin alphabet and then maybe in an invented conscript.3. Decide the basics for the grammar - Isolated? Agglutinated? Etc. Then make a list of ideas e.g. list of cases, list of tenses, particles etc. and then put them into words and affixes in your conlang (Deciding irregularities if wanted at this point). 4. Lexicon - create simple words first (also simple particles or affixes e.g. -ness, -able) and then build up and combine them into more complex vocabulary and compound words.

The first thing I do when creating a conlang is get punched in the face by inspiration. Usually in regards to some kind of writing system or phonological idea. So that's where I start: the phonology and the writing system. After that I get into verb tenses and forms (past, present, future, infinitive, etc). Then I'll create one verb for testing purposes (usually the word go, which in my strongest conlang is åjaa (AYE-aah or aye-AAH, depending on the circumstances). After playing around with verb forms for a bit, and making sure they fit together, I move on to the pronouns, since one can easily create a full body of pronouns in one sitting. Then I'll move on to pronoun modifications (like plural, possessive, etc.) and test those around for a little with various forms of my test verb and see if they all fit together. At this point I'll make a test noun (usually the word store, since it works well with go, which in my strongest conlang is purho (POOR-hoe) (please ignore the slanderous transliteration. lol). At this point, I'll get started on noun cases (nominative, genetive, accusative, etc.). Since my conlangs are all at least somewhat agglutinative, I usually end up making lots and lots of these, often inventing my own cases (examples being what I call the "equative" case, being an essive case denoting equality or sameness (purhoksi: the same store), and the "supraperlative" case, being an internal locative case denoting "throughout." (purholjeira: throughout the store)). And now, with our verb tenses and our test verb and our test noun, and our noun cases and our pronouns, we have enough to make a sentence (using my own conlang as an example): Purhovöl seiessa'åjaahinde: "Let's go to the store." Everything in that sentence had been made during the steps I provided. Purho is our test noun, -völ is our new allative case, sei is our first person pronoun, -essa is our plural, åjaa is our test verb, -hin is our present tense verb suffix, and -de is one of our newly created verb forms denoting a request or a command for the immediate future. You see that? Now we have a fully functioning sentence and it didn't take that long, did it? Now you have to get into adjectives and adverbs (be creative), methods of expressing various levels of sureness, etc., and make yourself a few hundred nouns and verbs. Presto.